1. Field of the Invention
The invention pertains to the field of pharmaceutical chemistry and provides a stereoselective fusion glycosylation process for preparing 2'-deoxy-2',2'-difluoronucleosides and 2'-deoxy-2'-fluoronucleosides.
2. State of the Art
The continued interest in the synthesis of 2'-deoxynucleosides and their analogues is reflected in their successful use as therapeutic agents in viral and cancerous diseases. A critical step in the synthesis of 2'-deoxynucleosides is the condensation of the nucleobase and carbohydrate to form the N-glycosidic bond. When the carbohydrate possesses a 2-hydroxy substituent, the substituent provides a substantial degree of 1,2-anchiomeric assistance, which facilitates stereoselective glycosylation. However, processes for synthesizing of 2'-deoxynucleosides are typically non-stereoselective and form a mixture of aloha and beta nucleosides.
Another type of condensation is fusion glycosylation, which is carried out in the absence of a solvent and at reaction temperatures sufficient to convert the carbohydrate and nucleosides base reactants to a molten phase. The original fusion glycosylation process was used to prepare purine nucleosides and involved reacting a peracylated sugar with a fusible purine base under vacuum and in the presence of a catalyst, such as p-toluenesulfonic acid. However, the glycosylation was not effective in condensing pyrimidine nucleosides because of their high melting points. In addition, the yields would vary widely and a broad range of anomerically mixed nucleoside products were produced.
T. Shimadate, et al., in Nippon Kagaku Zasshi, 81, 1440-1444 (1960) and Chem. Abstracts, 56, 11692 (1962), described a fusion glycosylation process involving reacting a peracetylated arabinofuranose and a purine base in the molten phase, under vacuum, in the presence of a p-toluenesulfonic acid catalyst. T. Shimadate, Nippon Kagaku Zasshi, 82, 1268-1270 (1961), described a similar fusion glycosylation process using an anhydrous zinc dichloride catalyst.
R. P. Hodge, et al., J. Org. Chem., 56, 1553-1564 (1991), described the preparation of deoxythymidine, deoxycytidine, deoxyadenosine and deoxyguanosine nucleosides containing deuterium at the C-1' position of the carbohydrate 1-chloro-3,5-ditoluoylribofuranose. The glycosylation was based on the synthesis described in Hubbard, et al., Nucleic Acids Res., 12, 6827 (1984). The preparation of 2'-deoxycytidine required converting a silylated uridine derivative to a silylated cytidine derivative by one of the three different methods. In each, substantial quantities of undesirable alpha-anomer nucleoside product formed. Also, isolating the small quantity of beta-anomer nucleoside obtained from the anomeric mixture proved to be difficult.
The synthesis of purine deoxynucleosides was carried out by the procedure described in Robbins, Nucleic Acids RES., 12, 1179 (1984), and involved reacting a sodium salt of a halopurine with 1-chloro-3,5-ditoluoylribofuranose. The sodium salts of purine bases were found to be much better nucleophiles than silylated pyrimidine bases.
U.S. Pat. No. 4,526,988, Hertel, illustrated a fusion glycosylation process for preparing 2'-deoxy-2',2'-difluoronucleosides which involves reacting a 3,5-bis(t-butyldimethylsilyloxy)-1-methanesulfonyloxy-2-deoxy-2,2-difluoro ribofuranose with 5-methyl-2,4-bis(trimethylsilyloxy)-pyrimidine at 150.degree. C.
Vorbruggen, et al., J. Org. Chem., 41, 2084 (1976) provided an outstanding development in the field of glycosylation and showed how nucleosides may be obtained from the Friedel-Crafts catalyzed reaction of a peracylated carbohydrate and silylated heterocycles in a solvent such as, 1,2-dichloroethane and acetonitrile. But when this process was applied to the synthesis 2'-deoxynucleosides, a 1:1 alpha to beta-anomeric mixture of nucleoside products was produced. Recent reports, for example, Grienyl, et. al., J. Med. Chem,, 28, 1679 (1985), have indicated that condensation reactions carried out in chloroform show a preference for beta-anomer nucleosides in a 3:1 beta to alpha ratio.
Some deoxynucleosides have been prepared in high yield from deoxyhalogenose with Friedel-Crafts catalysts, notably, 1-chloro-2-deoxy-3,5-di-p-toluoyl-alpha-D-erythropentofuranose; see, M. Hofer, Chem. Ber, 93, 2777 (1960). However, halogenoses are less stable thermally than peracylated carbohydrates and produce a 1:1 alpha to beta-anomeric mixture of nucleoside products. Walker, et al., Nucleic Acid Research, 12, 6827 (1984), used halogenose in condensation reactions to study the factors controlling the atomeric ratio of nucleoside products and found that beta-anomer nucleosides were formed exclusively from alpha-halo-carbohydrates via SN.sub.2 displacement. The corresponding alpha-anomer nucleoside contamination was determined to result from the anomerization of alpha-halo carbohydrate to beta-halo carbohydrate before the SN.sub.2 displacement reaction. Walker et al., found that by changing the solvent or catalyst higher yields of the desired beta-anomer nucleoside were produced.
R. P. Hodge et al., J. Org. Chem., 56, 1553 (1991), described preparing pyrimidine and purine nucleosides containing deuterium at the C-1' position by the method described by Walker, et al. 1'-Deuterium-2'-deoxycytidine was prepared by reacting a carbohydrate and silylated cytosine derivative but the reaction gave poor yields. However, the yield was significantly improved when silylated uridine derivatives were used.
The synthesis of 2'-deoxy-2'-fluoronucleosides advanced rapidly when a procedure for synthesizing 2-deoxy-2-fluoro-3,5-di-O-benzoyl-alpha-D-arabinosyl bromide was made available: see Tann, et. al., J. Org. Chem., 50, 3644 (1985) and Howell, et. al., J. Org. Chem., 53, 85 (1988). It was discovered that 2-deoxy-2-fluoro-3,5-di-O-benzoyl-alpha-D-arabinosyl bromide did not anomerize in dry acetonitrile over extended periods. Therefore, high yields of beta-nucleosides could be obtained from 2-deoxy-2-fluoro-3,5-di-O-benzoyl-alpha-O-arabinosyl bromide via SN.sub.2 displacement. Also, stereoselectivity of the nucleoside products could be achieved if either carbon tetrachloride or chloroform solvents was employed.
The formation of the N-glycoside bond in 2'-deoxy-2',2'-difluoronucleoside synthesis is much more difficult than in instances where the carbohydrate is 1,2-anchiomericly assisted or contains 2-deoxy-2-fluoro groups. The traditional carbohydrate leaving groups, such as those used in the Vorbruggen condensation method, acetate, chloride and bromide, render the carbohydrate inactive. In order to overcome this problem, Hertel, U.S. Pat. No. 4,526,988, described a modified version of the Vorbruggen condensation method that relied on more reactive sulfonate leaving groups attached to the carbohydrate to affect its reactivity. For example, hydroxy protected carbohydrates, such as 2-deoxy-2,2-difluoro-D-ribofuranose, containing a methanesulfonate, toluenesulfonate, ethanesulfonate, isopropanesulfonate or 4-methoxybenzenesulfonate leaving group at the C-1 position, were reacted with a protected nucleobase at temperatures of 50.degree. C. to 220.degree. C., in the presence of a high boiling solvent, such as dimethylformamide, dimethylacetamide and hexamethylphosphoramide. Hertel teaches that when carrying out the glycosylation reaction at elevated pressures, any convenient inert solvent, such as ethers, halogenated alkanes, and aromatics, can be used since the elevated pressure eliminates the loss of low boiling inert solvents due to evaporation. However, at reaction temperatures from room temperature to 100.degree. C., a catalyst such as trifluoromethanesulfonyloxysilane, is required. U.S. Pat. No. 4,965,374, Chou, et al. reports that Hertel's condensation method provides alpha-anomer stereoselectively and therefore forms a 4:1 alpha to beta atomeric ratio of nucleoside products and goes on to describe an improved procedure, based on the Vorbruggen condensation method, that employs a pivotal intermediate of 2-deoxy-2,2-difluoro-3,5-di-O-benzoyl-alpha-D-arabinosyl methanesulfonate. However, Chou's condensation method forms a 1:1 alpha to beta anomer mixture of nucleoside products.
Despite the preceding advances in nucleoside synthesis, there continues to be a need for a stereoselective fusion glycosylation process capable of efficiently producing beta- or alpha-anomer enriched 2'-deoxy-2',2'-difluoronucleosides and 2'-deoxy-2'-fluoron in increased yields.
Accordingly, one object of the present invention is to provide a stereoselective fusion glycosylation process for preparing beta or alpha-anomer enriched 2'-deoxy-2',2'-difluoronucleosides and 2'-deoxy-2'-fluoronu
Another object of the present invention is to provide a stereoselective fusion glycosylation process for preparing beta or alpha-anomer enriched 2'-deoxy-2',2'-difluoronucleosides and 2'-deoxy-2'-fluoronucleosides without the use of a catalyst.
Another object of the present invention is to provide a stereoselective fusion glycosylation process for preparing beta- or alpha-anomer enriched 2'-deoxy-2',2'-difluoronucleosides and 2'-deoxy-2'-fluoronucleosides in yields higher than those produced by conventional fusion glycosylation procedures.
Yet another object of the present invention is to provide a stereoselective fusion glycosylation process for preparing beta or alpha-anomer enriched 2'-deoxy-2',2'-difluoronucleosides that offers a means for isolating intermediates of the beta- or alpha-anomer enriched nucleosides as a crude product or acid addition salt such as a hydrochloride salt.
Other objects and advantages of the present invention will become apparent from the following description of embodiments.